A method for forming a desired image by developing an electrostatic latent image with a toner for developing electrostatic images, has been widely used. For example, in electrophotography, an electrostatic latent image formed on a photoconductor is developed with a toner made from colored particles optionally containing other particles of an external additive, carrier, etc. Then, the developed image is transferred onto a recording medium such as a paper or OHP sheet and then fixed to obtain a printed product.
Various kinds of properties as described below are required of toners.
First, high speed printing is required of electrophotographic copy machines, printers and so on, as well as low power consumption. In electrophotography, a process that involves particularly high energy consumption is a process of fixing a toner transferred onto a transfer material such as paper, that is, a so-called fixing process. In general, a heat roller at 150° C. or more is used for fixation, and energy consumption of the fixing process can be reduced by decreasing the temperature of the heat roller. To respond to high speed printing, it is needed to increase fixing temperature, so that the time required for the transfer material such as paper to pass through a fixing roller, is shortened. However, in this case, there is a problem of increase in energy consumption. To satisfy both the demands for low power consumption and high speed printing, it is increasingly essential to decrease toner fixing temperature.
Other properties required of toners include stable image properties even under various kinds of environments such as a high temperature and high humidity environment and a low temperature and low humidity environment (environmental stability) and no image deterioration even when making a large number of prints (printing durability).
In general, toners are broadly classified into a toner produced by a pulverization method and a toner produced by a polymerization method.
In the pulverization method, colored resin particles are produced by a method of pulverizing and classifying a solid colored resin product, which is obtained by melt-kneading a binder resin and a colorant or polymerizing a mixture containing a monomer and a colorant.
As the polymerization method, for example, there may be mentioned a suspension polymerization method in which colored resin particles are produced by forming and polymerizing droplets of a polymerizable monomer composition containing a polymerizable monomer and a colorant, and an emulsion polymerization aggregation method in which colored resin particles are produced by polymerizing an emulsified polymerizable monomer to obtain resin fine particles and aggregating the fine particles with a colorant, etc. While the form of the colored resin particles obtained by the pulverization method is not uniform, the form of the colored resin particles obtained by the polymerization method is close to a spherical form, and the particles have a small particle diameter and a narrow particle size distribution. Especially from the viewpoint of improving image quality such as image reproducibility and fineness, toners with a highly-controlled form and particle size distribution, like toners produced by the polymerization method (i.e., polymerized toner), have been increasingly used. Moreover, as a toner structure having both low-temperature fixability and heat-resistant storage stability, a so-called core-shell structure has been proposed, in which a resin having a low glass transition temperature is covered with a resin having a high glass transition temperature.
For example, in Patent Literature 1, a method for producing toner particles by a suspension polymerization method is proposed, in which, at first, polymer particles that have a glass transition temperature of 80° C. or less are formed as the core component (core particles), and a monomer that can form a polymer having a higher glass transition temperature than the polymer of the core component is added to keep polymerization reaction and form a coating layer of the shell component having a high glass transition temperature on the core component, thereby producing a capsule type toner.
In Patent Literature 2, a method for producing a thermocompression fixing capsule toner is proposed, which is composed of a hot-melt core material containing a thermoplastic resin and a colorant, and an outer shell covering the surface of the core material, the method having the steps of: dispersing a raw material for constituting the core material and a hydrophilic shell material in an aqueous dispersion medium; producing encapsulated particles composed of the core material covered with the hydrophilic shell material by in situ polymerization and using them as precursor particles; adding at least a vinyl polymerizable monomer and a vinyl polymerization initiator to the aqueous suspension of the precursor particles to be absorbed into the precursor particles; and then polymerizing the monomer component of the precursor particles.
However, the toners of these patent literatures are insufficient to provide toners for developing electrostatic images, which have both heat-resistant storage stability and low-temperature fixability.